AC-DC Switched mode power convertors, for converting power from an alternating current source, such as a mains supply, to a direct current (DC) output, are well known. Such converters have been used in domestic environments in particular for several years, for instance to provide power to consumer electronic equipment such as televisions, video players, computers and the like. The recent and growing trend for incandescent lighting to be replaced by energy-efficient lighting such as compact fluorescent lighting or LED-based lighting is anticipated to result in a significant increase in the importance of AC-DC switched mode power converters for domestic applications. With the increasing use of switched mode power converters, has come an increasing scrutiny of their impact on the mains supply.
A measure of the impact of any load on a mains supply is the so-called power factor measurement. The power factor of a load is defined as the ratio of the real power to the root mean square value of the voltage-current product. An entirely real load which draws power smoothly and consistently across the whole of the mains cycle or mains half cycle has a power factor of unity; lower power factor is indicative of either a imaginary load (inductive or capacitive) or un-even drawing of power from the mains. The latter, in particular, can result in distortion of the supply from the ideal AC profile, and is associated with the presence of higher harmonics of the fundamental mains frequency.
United States patent application US20051057237 discloses a power factor controller in which the load and line regulation circutory is separate from one another and in which the line regulation is provided with a modulator in which the switching frequency is inversely proportional to the square of the line voltage.
Switched mode power supplies in general are susceptible to disrupting the mains, and typically have a power factor which is significantly less than unity. The high-frequency switching of switched mode power supplies make them particularly susceptible to low power factors. Moreover, when switch mode power supplies are operated in a relatively low-power mode (such as may be the case for supplying the power to electronic consumer electronic equipment in standby mode, or to an LED lighting circuit which is dimmed), the power factor may often fall lower than an acceptable standard.
In order to maintain a high power factor across a range of operating conditions, some known AC-DC converters are configured to operate in boundary conduction mode with Ton control. In the particular case of a flyback converter, this means the on-time of the switch is predetermined, during which the current in the inductor rises linearly. When the switch is opened, the current falls, as power is transferred to the load; when the current falls to zero (typically at the first valley in a resulting resonance) the switch is re-closed to restart cycle. Although this mode of operation is straightforward to establish, the efficiency is not optimal.
Moreover, in order to provide for more sophisticated control, without a commensurate increase in the number and thus cost of components, there is an increasing trend towards digital control rather than conventional analogue control.
It would be desirable to provide an alternative method of controlling an an AC-DC switched mode power converter which may be operable with high power factor, and is compatible with digital control.